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Jun 5, 2013 - A 3-year survey was conducted in Iowa to characterize the distribution and frequency of species of Fusarium associated with soybean roots.
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Distribution and Frequency of Fusarium Species Associated with Soybean Roots in Iowa M. M. Díaz Arias, G. P. Munkvold, M. L. Ellis, and L. F. S. Leandro, Department of Plant Pathology and Microbiology, Iowa State University, Ames

Abstract Díaz Arias, M. M., Munkvold, G. P., Ellis, M. L., and Leandro, L. F. S. 2013. Distribution and frequency of Fusarium species associated with soybean roots in Iowa. Plant Dis. 97:1557-1562. A 3-year survey was conducted in Iowa to characterize the distribution and frequency of species of Fusarium associated with soybean roots. Ten plants were collected from each of 40 to 57 fields each year at V2 to V5 and R3 to R4 soybean growth stages. Fusarium colonies were isolated from symptomatic and symptomless roots and identified to species based on cultural and morphological characteristics. Species identification was confirmed by amplification and sequencing of the translation elongation factor (EF1-α) gene. Fifteen species were identified; Fusarium oxysporum was isolated most frequently, accounting for more than 30% of all isolates. F. acuminatum, F. graminearum, and F. solani were also among the most frequent and widespread species.

Eleven other species were recovered from few fields, accounting for less than 10% of all isolates in a given year. No consistent trends were observed in geographic distribution of species. Variability in species frequency was found between soybean growth stages. Fusarium oxysporum was recovered at higher frequency during vegetative stages (40%) than reproductive stages (22%). Conversely, species such as F. acuminatum, F. graminearum, and F. solani were recovered more often from reproductive-stage plants. No significant differences in species composition were observed among fields differing in tillage practices and row spacing.

Fungi in the genus Fusarium cause several diseases of soybean (Glycine max (L.) Merr.), including Fusarium wilt, caused by Fusarium oxysporum Schltdl. (2); sudden death syndrome (SDS), caused in North America by F. virguliforme O’Donnell & T. Aoki (1); as well as seed and seedling diseases and root rot, caused by numerous species (7,28,33,37). Fusarium root rot is a common and widespread disease in the United States (40). At least 19 Fusarium spp. have been isolated from soybean roots in different soybean production areas (4,5,10,23,24,28,32). Previous studies have demonstrated that F. oxysporum and F. solani are generally the most common species in soybean tissues and the main causal agents of soybean root rot in the United States (9,13,24,28) and other countries (3,20,41). Affected plants may exhibit poor or slow emergence and root symptoms, including dark brown lesions, especially in the lower portion of the root system, or decay of the entire taproot (28). Several studies on occurrence, distribution, pathogenicity, and cultivar resistance have been conducted for F. oxysporum and F. solani (13,14,22,29) but there is limited work on the other Fusarium spp. isolated from soybean roots (4,9,24). These reports as well as field observations indicate that colonization of soybean roots by several of these species is common; however, there is very limited information about their pathogenicity on soybean and their impacts on root health and yield. Fusarium root rot was first reported as a problem in soybean in Iowa in 1953 by Dunleavy (30). The disease was initially attributed to F. orthoceras Appel & Wollenw. but later it was documented that F. oxysporum was the predominant fungus isolated from symptomatic soybean roots (14,30). Several other Fusarium spp. have

been consistently isolated from soybean plants in Iowa fields. Nyvall (30), who conducted a 3-year study, isolated Fusarium spp. from pods, stems, and roots from soybean plants collected in nine fields in central and north Iowa. In this study, F. oxysporum was the predominant colonizer of roots and soil, followed by F. solani. Another important soybean root rot pathogen in Iowa is F. virguliforme, the cause of SDS, and various studies have been done on the biology, epidemiology, and cultivar resistance for this disease (16,27). Although Fusarium root rot has long been known as a common soybean disease, the importance of this disease to Iowa soybean production in not fully understood. In addition, there is a lack of information about the species involved and their abundance and geographical distribution around the state. Therefore, the objectives of the present study were to (i) characterize the diversity and frequency of Fusarium spp. associated with soybean roots in Iowa and (ii) determine the geographic distribution of Fusarium spp. in Iowa during vegetative and reproductive stages of soybean growth.

Corresponding author: L. F. S. Leandro, E-mail: [email protected] * The e-Xtra logo stands for “electronic extra” and indicates that two supplementary figures and one supplementary table are available online. Accepted for publication 5 June 2013.

http://dx.doi.org/10.1094 / PDIS-11-12-1059-RE © 2013 The American Phytopathological Society

Materials and Methods Sample collection. Soybean roots were collected during the 2007, 2008, and 2009 growing seasons across the nine Iowa crop reporting districts (Fig. 1). In collaboration with 12 Iowa State University extension specialists, 142 soybean fields from 98 Iowa counties were surveyed over the 3-year period (Fig. 1). All the fields sampled had corn as the previous crop, with the exception of three locations. Ten plants with as much of the root system as possible were collected arbitrarily from each field at both vegetative (V2 to V5) and reproductive (R2 to R4) growth stages (18); plants were dug and soil was gently shaken from roots before plants were placed in plastic bags. Samples were shipped overnight to the Iowa State University Plant & Insect Diagnostic Clinic or The Seed Science Center and stored at 4°C until processed. Some fields were sampled only at one growth stage and some counties were sampled in more than 1 year. In total, 101 samples was collected from 55 fields in 50 counties in 2007, 77 samples from 40 fields in 40 counties in 2008, and 84 samples from 47 fields in 40 counties in 2009 (Fig. 1). In each field, sampling date, location (geographic coordinates), and soybean growth stage were documented. Row spacing (19.1, 38.1, and 76.2 cm) and tillage type (conventional Plant Disease / December 2013


till, no till, and reduced tillage) were also recorded in order to determine possible differences in species frequency in relation to crop production practices. For each year, statewide average air temperature and precipitation from May to July were obtained from Iowa Environmental Mesonet, Department of Agronomy, Iowa State University (http://mesonet.agron.iastate.edu/climodat/). Isolation and identification of Fusarium spp. Roots were rinsed under running tap water for 10 min to remove soil and debris from the field. Fungi were isolated from roots by arbitrarily excising two 1-cm-long root pieces from each plant, one from the taproot and one from the lateral root tissue. Root pieces were surface disinfected in 0.5% NaOCl for 2 min, rinsed three times in sterile distilled water, and blotted dry. Root pieces were embedded in antibiotic-amended Nash Snyder medium (per liter: 15 g of peptone, 1 g of KH2PO4, 0.5 g of MgSO4⋅7H2O, 20 g of agar, 0.75 g of pentachloronitrobenzene (PCNB), streptomycin sulfate solution at 33 mg/ml, and neomycin sulfate solution at 40 mg/ml) and incubated at room temperature (25 ± 2°C) for 7 days under fluorescent light. Numbers of root pieces with putative Fusarium colonies were recorded for each sample. Colonies were transferred into 1.5-ml centrifuge tubes containing potato dextrose broth (PDB) by removing a small piece of mycelium from the colony margin, maintained on a rotary shaker at 200 rpm for 3 to 5 days to allow colony growth, and then stored at 4°C for future species identification. From each set of 10 plants sampled per field, 1 to 20 putative Fusarium isolates were recovered. In total, 2,193 isolates (793 isolations in 2007, 588 in 2008, and 812 in 2009) were collected. For identification to species level, 1 to 15 isolates were selected arbitrarily from within each field sample, for a total of 1,640 isolates. Each isolate was transferred from PDB to carnation leaf agar (CLA) (25) amended with KCl (15 g of agar and 6 g of KCl per liter) and antibiotic-amended potato dextrose agar (PDA) (streptomycin sulfate solution at 33 mg/ml, neomycin sulfate solution at 40 mg/ml, and 39 g of PDA per liter), and cultures were incubated for 7 to 20 days at room temperature under fluorescent light. Each of the 1,640 isolates was examined microscopically and identified to species according to the system of Leslie and Summerell (25). Species were identified based on the most distinctive morphological characteristics, including macroconidia and microconidia, production of mesoconidia (e.g., F. semitectum), conidial arrangements (singly, false heads, and chains), nature of the conidiogenous cells (monophialides [e.g., F. oxysporum] or polyphialides [e.g., F. proliferatum]), conidiophore length (e.g F. solani versus F. oxysporum), formation and arrangement of chlamydospores (e.g. F.

Fig. 1. Location of 142 soybean fields surveyed in 98 Iowa counties during the 2007, 2008, and 2009 growing seasons across the nine Iowa crop reporting districts: Northwest (NW), North Central (NC), Northeast (NE), West Central (WC), Central (C), East Central (EC), Southwest (SW), South Central (SC), and Southeast (SE). 1558

Plant Disease / Vol. 97 No. 12

equiseti), production and color of sporodochia on CLA, and colony appearance and pigment formation on PDA (Supplementary Figure 1). Of the 1,640 isolates identified, 1,254 were species of Fusarium based on cultural and morphological characteristics. Species identities were confirmed for a subset of 170 isolates by amplification and sequencing of the elongation factor 1-α (EF1-α) gene region. The isolates were selected to represent all the distinguishable and unknown morphological groups that were observed. Single-spore cultures were obtained for each of the 170 isolates used for molecular identification. All isolates were grown on PDA at room temperature for 7 days. The mycelia were harvested, 50 to 100 mg of mycelia was placed into a 1.5-ml centrifuge tube, and then each sample was lyophilized for 2 days and immediately crushed with a sterile pestle in the tube. Genomic DNA was extracted using the cetyltrimethylammonium bromide procedure (19). DNA was quantified using a spectrophotometer and then stored at –20°C until processed. Primers pair used for amplification and sequencing were ef1/ef2 (5′ATGGGTAAGGAGGACAAGAC3′/5′GGAAGTACCAGTGATCATGTT-3′) and ef1/ef22 (5′-AGG AACCCTTACCGAGCTC-3′), respectively (15,31). Polymerase chain reactions (PCRs) were conducted in 20-µl volumes. Each reaction tube contained approximately 1 µl of a 1-ng/µl DNA template, 3.2 µl of 10× Titanium Taq buffer (35 mM MgCl2), 9.7 µl of sterile distilled water, 2 µl of 2 mM dNTPs, 2 µl each of forward and reverse primers (2.5 µM), and 0.1 µl of DNA Taq polymerase (Titanium Taq). The thermal cycling parameters were initial denaturation at 94°C for 1 min followed by 31 cycles consisting of denaturation at 94°C for 30 s, annealing at 53°C for 45 s, and extension at 72°C for 1 min. A final extension at 72°C for 5 min was done at the end of the amplification. PCR products were purified adding 2 µl of ExoSAP-IT reagent (Affymetrix Inc.) in 5 µl of PCR. PCR products were sequenced at the Iowa State University DNA and Sequencing Facility. DNA sequences were trimmed to 400 to 500 bp using CodonCode Aligner 4.1.1 (CodonCode Corp.). Sequences were submitted for BLAST searches for comparison to known DNA sequences in the National Center for Biotechnology Information (NCBI) (http://ncbi.nlm.nih.gov) and the FUSARIUM-ID/FUNCBS databases in Fusarium MLST (http://www.cbs. knaw.nl/Fusarium). Sequences were also compared with reference strains from The Fusarium Laboratory Manual (25) using BLASTn from NCBI (http://ncbi.nlm.nih.gov). The accession numbers for the reference strains and the percent identity of the isolates to those strains are listed in Supplementary Table 1. Data analysis. For each Fusarium sp., relative frequency and incidence were calculated using the total number of Fusarium isolates and root pieces, respectively, from all fields sampled in a given year. The relative frequency (F) of Fusarium spp. was evaluated using the formula F = 100 × (n/N), in which n = the number of isolates of each species and N = the total number of isolates of all species (34). Incidence of each species was defined as the percentage of all root pieces which yielded that species (26), and was estimated as IS = 100 × (rs/ri) × ra/R, in which rs = number of root pieces yielding a species, ri = number of root pieces selected for identification, ra = number of root pieces yielding any isolate, and R = total number of root pieces sampled. Prevalence of Fusarium spp. was calculated at the state level, using both field and county as sampling units, by dividing total number of fields or counties in which each Fusarium spp. was present by total number of fields or counties sampled around the state. In addition, Fusarium spp. prevalence was calculated at the district level as the total number of fields in which each Fusarium spp. was present (× 100) divided by the total number of fields sampled per district. Frequencies of isolation were compared for the overall data set to test for differences among species, and between crop growth stages, tillage practices, row spacing, and root type (lateral versus taproot). Analyses of variance were conducted using the general linear model procedure (PROC GLIMMIX) of SAS (version 9.2, SAS Institute) using year as replication (random factor). Differences among means were compared using the Tukey-Kramer test and considered significantly different if P ≤ 0.05. Incidence and

prevalence data were analyzed for the overall data set, as well as for tillage and row spacing, using the same model as above.

Results Fifteen species were identified, including F. acuminatum, F. armeniacum, F. avenaceum, F. equiseti, F. graminearum, F. oxysporum, F. poae, F. proliferatum, F. semitectum, F. solani, F. sporotrichioides, F. subglutinans, F. tricinctum, F. verticillioides, and F. virguliforme (Fig. 2). Morphological characteristics such as mycelium appearance, growth rate, pigment, and spodorochia formation varied among isolates within a species, particularly for F. armeniacum, F. semitectum, F. oxysporum, and F. solani. Strains belonging to the F. oxysporum complex showed the widest variation in culture morphology on PDA. Frequency of isolation of Fusarium spp. Mean frequency of isolation averaged over years (data not shown) varied among Fusarium spp. (P < 0.0001). Frequency of isolation of F. oxysporum, F. acuminatum, and F. solani was greater (P < 0.0001) than all the other species. F. oxysporum showed significantly higher frequency (32%) than all other species, except F. acuminatum (23.9%). Frequency of F. solani was 21.5% and did not differ from F. acuminatum but did differ from all other species which were isolated with a frequency of 8% or lower. There were no differences in frequency among the remaining species. The main trends observed in the data plotted per year (Fig. 2) were similar to those observed for data combined over years. F. oxysporum was the most frequently recovered species, followed by F. acuminatum and F. solani, each ranging from 12 to 37% of the isolates (Fig. 2). Other species such as F. graminearum, F. proliferatum, and F. sporotrichioides represented 1 to 12% of the isolates. Remaining species such as F. armeniacum, F. avenaceum, F. equi-

Fig. 2. Frequency of isolation (%) of Fusarium spp. from soybean roots in Iowa in 2007, 2008, and 2009.

seti, F. semitectum, F. subglutinans, F. poae, F. tricinctum F. verticillioides, and F. virguliforme were less frequent (